(1) Field of the Invention
This invention relates to a magnetic recording medium comprising a substrate disc having formed thereon a magnetic recording layer, and a process for producing the magnetic recording medium. More particularly, it relates to an improvement in a magnetic recording medium comprising a substrate disc having formed thereon a magnetic recording layer, said substrate disc having a multiplicity of microscopic bumps on the surface thereof formed by texturing the substrate surface by focusing a laser beam; and to an improvement in a process for producing the magnetic recording medium.
The magnetic recording medium of the present invention exhibits good CSS (contact start-stop) characteristics and a reduced head noise, and thus, has an enhanced durability against wear due to sliding contact of a magnetic hard disc (hereinafter abbreviated to "HD") with a magnetic head.
(2) Description of the Related Art
The recent progress of increasing the density of magnetic recording medium is remarkable. It has heretofore said that the rate of increase in the recording density of hard disc drives (hereinafter abbreviated to "HDD") is about 10 times per 10 years, but now it may be said that the rate of density increase is about 100 times per 10 years.
In HDD, there is adopted mainly a Winchester system, i.e., CSS (contact start--stop) system involving a basic operation of a sliding movement due to contact of HD with a magnetic head, floating of the head above HD and then a sliding movement due to contact of HD with the head. The CSS system has contributed in no small measure to the amazing progress in increase of recording density. However, this system has invited an attractive problem of tribology. More specifically, the amazing progress in increase of recording density has invited an increase of revolving speed of the disc and a reduction of a flying height of the magnetic head. Therefore, there is now an increasing demand for improving head-and-disc wear characteristics and stability in sliding movement, and for enhancing smoothness of the HD surface.
The key to the improvement of head-and-disc-wear characteristics lies in an increase of the tenacity of materials and a lowering of friction coefficient or an enhancement of lubrication. As regards HD, attempts have been made for lowering the friction coefficient by rendering rough the HD surface, and for coating the HD with a protective coating material such as diamond-like carbon (DLC) or a coating lubricating agent. The surface treatment for lowering the friction coefficient is referred to as "texturing treatment" and is intended to effectively reduce the contact area in the CSS system whereby the head-and-disc wear characteristics are improved. The texturing treatment comprises forming a multiplicity of bumps having peaks and indentations or valleys with predetermined height and depth on the textured HD surface, and now the texturing treatment is an essential step for the HD production.
The texturing treatment greatly depends upon the particular material of the substrate disc. For example, in the case of an aluminum disc blank having a nickel-phosphorus (NiP) coating, the surface roughening is generally effected by mechanical polishing using an abrasive grain. In the case of a glass substrate, an etching technique utilizing lithography or a combination of lithography with printing have been proposed and some of the proposed techniques have been practically adopted.
In the texturing treatment, there are problems inconsistent with each other, namely, it is difficult to precisely control the surface roughness with an enhanced production efficiency. For example, the mechanical abrasion has technical problems of over-abrasion or burr occurrence, and blurring of textured areas, and the lithographic etching has a problem that the production steps are complicated.
In recent years a texturing treatment utilizing laser beam such as laser ablation and laser etching have attracted attention (see, for example, U.S. Pat. No. 5,062,021 and Japanese Unexamined Patent Publication (hereinafter abbreviated to "JP-A") 62-209,788, 3-272018 and 7-182655). The laser beam texturing is advantageous in that, first, the surface roughness of substrate can be precisely controlled, namely, the height, spacing and location of microscopic bumps created by laser beam focusing can be desirably controlled, and, secondly, the production steps are carried out in a dry state without use of any liquid and thus the HD production can be conducted without contamination of the working environment. For example, U.S. Pat. No. 5,062,021 discloses creation of crater-like bumps each comprised of a circular rim surrounding a pit and having a diameter of 2.5 to 100 .mu.m and a spot spacing of 12.7 to 25.4 .mu.m on an NiP-coated aluminum substrate by using a Q-switched pulse oscillating Nd-YAG laser with a wavelength of 1,064 nm and an oscillation frequency of 12 KHz.
But, the laser beam texturing has problems such that, when the preset height and spacing of bumps are not adequate, the adsorption of the magnetic head onto the disc surface and the CSS characteristics are markedly deteriorated.
The spacing between adjacent microscopic bumps in the circumferential direction of the substrate disc is usually constant, but natural oscillation undesirably occurs between the magnetic head and the bumps due to the bump spacings and the linear speed of the rotating disc in the circumferential direction thereof. The frequency of natural oscillation equals to integral number times of the circumferential speed/bump spacing ratio. Further, the magnetic head has a frequency of natural oscillation, and, when the frequency of natural oscillation of the magnetic head, is equal to integral number times of the frequency of natural oscillation, resonance occurs.
The occurrence of resonance leads to reduction in flying stability of a glide head for inspecting unacceptable bumps on the disc surface with the result of increase of a head noise and failure of inspecting the normal glide height, and further, it leads to deterioration of CSS (contact start-stop) characteristics.
When a laser beam is focused with a constant pulse repetition frequency, the spacing between adjacent microscopic bumps in the circumferential direction of the substrate disc varies in the radial direction. Namely, the bump spacings in outer circumferences of the substrate disc are larger than those in inner circumferences thereof. However, the bump spacings in the same circumferences are equal and thus, the above-mentioned problems occur.